This invention relates to a photoplotter and a related process for exposing graphics on a sheet of photosensitive material, such as a sheet of photographic film, a mat or plate used in photolithographic printing, a photosensitive-resist covered plate for use in making printed circuitboards, or the like, and deals more particularly with such a photoplotter and process wherein the details of the exposed graphic are defined by a light valve device interposed between the photosensitive surface and a source of light and having a large number of subareas or pixels individually controllable as to their light transmissivity.
As used herein, the term "photoplotter" refers to a device which functions to create, compose and/or plot a graphic during the exposure process from digital information or similar signals supplied by an associated control device such as a computer or numerical controller.
Photoplotters of various different kinds are well known and widely used for exposing graphics on different photosensitive materials. For example, they are commonly used for exposing drawings and/or text on a sheet of photographic film. After development of the film it may become a photographic positive for direct use or it may it become a photographic negative or transparency usable as a mask in a subsequent photoexposure step, such as a mask for making a printed circuitboard or an integrated circuit chip. Known photoplotters are also often used in photolithographic printing processes for exposing photosensitive printing mats or plates and in circuitboard manufacturing processes for directly exposing a layer of photo-resist on a circuitboard blank.
In an early form of photoplotter, such as shown by U.S. Pat. No. 3,330,182, a spot of light is projected onto the photosensitive surface and is moved in X- and Y-coordinate directions relative to the surface to generate a graphic or artwork on the photosensitive sheet, the process involved being analogous to that of writing on a sheet of paper with a pencil or pen except for the pencil or pen being replaced by the light spot.
In another common form of photoplotter, such as shown in U.S. Pat. No. 4,354,196, a light beam, usually a laser beam, is swept across the surface of a photosensitive material in raster fashion and selectively turned on or off for each increment of displacement, or pixel, along the length of each scan line, the resulting graphic being therefore exposed pixelwise generally similar to the way a picture is created on a television screen.
U.S. Pat. Nos. 3,689,932 and 3,724,347 show photoplotters wherein spots or figures created on the screen of a cathode ray tube are used in exposing graphics on a photosensitive material. U.S. Pat. No. 3,695,154 shows a photoplotter which "writes" on a sheet of photosensitive material with a projected light spot, the light spot being additionally variable in size and shape during the writing process to achieve various benefits.
All of the above-mentioned prior art photoplotters have characteristics which individually suit them well for various specific applications, but as a general rule they all tend to require considerable time for the exposure of a reasonably complicated graphic and have other characteristics making them difficult to use or unsatisfactory in certain instances.
The general object of this invention is therefore to provide a new type of photoplotter generally capable of exposing graphics at greater speed than known photoplotters and having characteristics making it more desirable than presently known photoplotters in many applications.
Other advantages and objects of the invention will be apparent from the following description of the preferred embodiments of the invention.